Project summary Poly(ADP-ribose) (PAR) polymerase-1 (PARP-1) plays a pivotal role in DNA damage response. It can be activated either by the DNA alkylating agents abundant in our environment, or the byproducts of the cellular oxidative stress or toxicity. In response to mild DNA damage, PARP-1 facilitates DNA repair process. Blockage of PARP-1 activity sensitizes cancer cells to death. This concept is well supported by PARP inhibitor olaparib used in clinic for cancer therapy. In contrast, in response to severe DNA damage, excessive activation of PARP-1 causes the large DNA fragments and caspase-independent cell death designated parthanatos, which occurs in many organ systems and is widely involved in different neurologic and non-neurologic diseases, including ischemia-reperfusion injury after stroke and myocardial infarction, glutamate excitotoxicity, neurodegenerative diseases, inflammatory injury, reactive oxygen species?induced injury. This type of cell death is profoundly prevented by pharmacological inhibition or genetic deletion of PARP-1. The importance of PARP-1 in cell death has also been appreciated in the cancer field as alkylating agents have been used in chemotherapy to kill cancer cells. Therefore, these PARP-1 studies raise a huge knowledge gap how PARP-1 signaling is regulated in DNA damage or oxidative stress, leading to either DNA repair/cell survival or DNA damage/cell death. Previous works from the PI have revealed the molecular mechanisms by which PAR triggers apoptosis inducing factor (AIF) release from the mitochondria and translocation to the nucleus, leading to PARP-1 dependent cell death (parthanatos). Recently PI further made an important discovery by identifying macrophage migration inhibitory factor (MIF) as a novel nuclease and an executioner in parthanatos, which cleaves genomic DNA into large fragments and causes neuron and cancer cell death. However, many fundamental questions, including 1) how MIF nuclease activity is regulated in response to DNA damage and oxidative stress in neurons and cancer cells; 2) how PARP-1 signaling in DNA damage is regulated and switched between cell death and cell survival in neurons and cancer cells; 3) how to effectively interfere PARP- 1 signaling in DNA damage to prevent excess neuron loss but enhance cancer cell death, have not yet been answered. The goals of this MIRA project are to obtain a comprehensive molecular understanding of PARP-1 signaling in DNA damage and to discover how PARP-1 signaling can be manipulated in response to mild or severe DNA damage thereby preventing neuronal cell death but enhancing tumor cell death. If successful, the knowledge achieved from these studies will directly impact the clinical application of PARP inhibitors for treatment of cancer or stroke. Moreover, the comprehensive understanding of the regulatory networks of PARP-1 signaling may also lead to the development of novel therapeutic strategies for the treatment of human diseases caused by PARP-1 activation.